The present invention relates to a method of forming a protective film to prevent a nitride read only memory cell charging, and more particularly, to a method of forming a nitride read only memory cell that the protective layers are formed in the inter-level dielectrics (ILD)/inter-metal dielectrics (IMD) layer.
Referring to FIG. 1, it shows a cross-sectional view of the structure of the conventional nitride read only memory cell. The forming of the nitride read only memory cell, firstly the active area is defined on the substrate 100 by using photolithography and such methods as the wet etching. The phosphorous ions (Pxe2x88x92) are doped into the substrate 100 by using the ion implantation to form the channel 124. The first oxide layer 102, the nitride layer 104, and the second oxide layer 106 are deposited on the substrate 100 in turn, and the nitride layer 104 is located between the first oxide layer 102 and the second oxide layer 106. The first oxide layer 102, the nitride layer 104, and the second oxide layer 106 are defined by using the photolithography and the etching process to form an oxide/nitride/oxide (ONO) structure 108 and expose the substrate 100.
The polysilicon layer 110 is deposited to cover the second oxide layer 106, and the silicide layer 112 is deposited to cover the polysilicon layer 110, then the polysilicon layer 110 and the silicide layer 112 are defined to expose the second oxide layer 106 by using photolithography and the etching process similarly, so that the gate 114 is formed. Next, a material layer, such as the tetra-ethyl-ortho-silicate (TEOS), the silicon dioxide (SiO2), or the silicon nitride (Si3N4) etc., is deposited, such as by the chemical vapor deposition (CVD) to cover the substrate 100, the second oxide layer 106, and the silicide layer 112. The material layer is defined by using photolithography and the anisotropic etching process to form the spacer 116.
Subsequently, the heavy highly concentrated doping and great depth is executed on the substrate 100 by using the structure consisted of the spacer 116 and the gate 114 as the mask, and the phosphorous (P) or the arsenic (As) that has greater solid solubility to the silicon (Si) as the ion source, so that the drain 122 and the source 126 are formed. An insulated layer 118 is deposited to cover the substrate 100, the spacer 116, and the silicide layer 112, and the ILD/IMD layer 120 is deposited to cover the insulated layer 118. Developed to this present, the nitride read only memory cell structure is completed.
For the following process, such as ultra-violet light or plasma usually penetrates through the nitride read only memory device to excite the atoms, so that the nitride read only memory device electrical instability results and damages the nitride read only memory device, or increases the ion mobility to cause the charge gain during the process, enhancing the threshold voltage, and affecting the stability of the device.
According to the conventional method of forming the nitride read only memory cell, there is no protective structure formed, so on the following process, the nitride read only memory cell may be penetrated through by the ultra-violet light or the plasma, and may increase the ion mobility to cause the charge gain, so that nitride read only memory cell damage will result and also result in the nitride read only memory cell electrical instability.
Accordingly, one aspect of the invention is to provide a method of forming the protective form to prevent a nitride read only memory cell charging. For nitride read only memory cell formation, the method of the present invention forms the protective layers to avoid the ultra-violet light and the plasma penetrating through the nitride read only memory device, to prevent the ion mobility increasing, and to keep the electricity""s stability.
Another aspect of the invention is to provide a method of forming the protective film to prevent a nitride read only memory cell charging. For nitride read only memory cell formation, the method of the present invention forms one or a plurality of protective layers in the ILD/IMD layer to avoid the charge gain during the process, increasing the ion mobility, and the ultra-violet light or the plasma penetrating, so to enhance the electricity""s stability of the device, decrease the nitride read only memory device""s threshold voltage, and expand the threshold voltage range.
For at least the foregoing aspects discussed above, the present invention provides a method of forming the protective film to prevent a nitride read only memory cell charging. The present invention""s method adds one or a plurality of protective layers, such as the silicon nitride (SiNx) layer or the silicon-oxy-nitride (SiON) layer, and the protective layers can resist the ultra-violet light illumination and the invading plasma, so that the nitride read only memory device cannot be excited to discharge in order to the ultra-violet light and the plasma, and decrease the device""s threshold voltage, so the device""s electricity can be controlled and kept constant.